Powdery mildew mostly tame, but worthy of watching

Temperatures are beginning to warm up across the state following something of a roller coaster ride earlier in the season.

PCAs and growers who use the UC Davis Powdery Mildew Risk Assessment Index (RAI) are keeping a close watch now that numbers are starting to increase in some areas. However, problems have been mostly avoided at this point.

“The grapes in our country look great so far,” says Maxwell Norton, Merced County farm advisor. “Temperatures have favored powdery mildew, but most growers are maintaining tight programs.”

Earlier weather caused a few other problems in Merced County, according to Norton. “We had a few vineyard blocks suffer significant crop loss from an unexpected localized frost a few weeks ago, but they were isolated incidents.”

Along the central coast, powdery mildew has been non-existent so far, according to Charlie Widle, PCA with Western Farm Service at Santa Maria.

“Thus far, we’ve seen zero incidence, and that’s what we’re hearing from the Paso Robles area as well,” he says. “If the temperatures warm up, we’ll be looking for it a little more closely, but so far, no problems to report.”

Preventive programs are holding the disease in check. Much of the industry is using less and less sulfur early in favor of Stylet oil followed by a fairly well-loaded arsenal of fungicides as the season progresses, according to Widle.

“We’ve got a lot to choose from. Stylet oil early in the season has definitely become more popular as an alternative to sulfur for the first line of defense. Some of the winemakers have become more opposed to the use of sulfur, which is driving down its use overall.

“I think Stylet oil works better, and it has the advantage of some suppressant activity on vine mealybugs, mites, and leafhoppers.”

If conditions become more conducive to powdery mildew, however, PCAs and growers will likely tighten treatment intervals and turn increasingly to the heavier guns to keep the problem at bay.